The present invention relates to a defect-review SEM (scanning electron microscope) best adapted to inspect contact holes formed during fabrication of semiconductor devices.
It is quite important to enhance the yield in fabricating semiconductor devices. It is known that factors deteriorating the manufacturing yield include random factors and process factors.
Random factors are associated with various semiconductor process factors concerned with semiconductor fabrication environments, such as the cleanliness of the cleanroom, generation of dust from the wafer conveyance system and from steppers, removal of substances peeled from etchers, and insufficient washing by wafer cleaning equipment. Therefore, the random factors are uncertain and attributed to dust produced by factors that are difficult to forecast.
Process factors are produced by deviations of actually fabricated patterns and structures from ideal chip linewidths, chip arrays, and chip structures based on semiconductor designs. To solve the problems of the process factors, tools for inspecting patterned wafers are installed in semiconductor cleanrooms for individual process steps.
Among inspections of the process factors, inspections of contact holes have attracted attention recently. For these inspections, optical inspection systems and defect-review SEMs have been developed.
An optical inspection system can inspect dust, foreign matter, and defective metallization patterns up to a feature size of 0.2 xcexcm by various methods. For example, scattering of light is employed. In another method, an electrical signal is obtained by detection of light and passed through a spatial filter to process the information. In a further method, features are extracted from optical images by pattern matching. In recent years, inspection equipment capable of inspecting contact holes having diameters of 0.1 to 0.05 xcexcm has been announced and attracted attention. However, the minimum contact hole diameter which can be inspected precisely is not obvious in practice.
Users utilize wafer pattern dimension metrology and inspection tools that are normally used to inspect contact holes using an electron beam. In this case, it has been reported that aspect ratios of about 3 to 5 could be inspected at a diameter of 0.2 xcexcm. However, defective apertures of contact holes smaller than 0.2 xcexcm and having aspect ratios of 10 to 15 cannot be inspected.
In recent years, a dedicated contact hole inspection system capable of inspecting contact holes having diameters of up to 0.1 xcexcm by sharply focusing the electron beam of a scanning electron microscope has been developed.
FIG. 1 schematically shows this inspection system having an electron gun (not shown) emitting an electron beam 1 that is accelerated. The beam 1 is sharply focused onto a semiconductor wafer sample 3 having a diameter of 8 to 12 inches by an electromagnetic lens 2. The beam is scanned by an electromagnetic deflector 4. A negative bias voltage (e.g., 19 kV) is applied to the sample to decelerate the beam down to 0.5 to 1.5 kV. The beam 1 is focused by the electromagnetic lens 2 to a diameter of less than 0.05 xcexcm on a pattern formed on the wafer sample 3.
The electromagnetic deflector 4 scans the electron beam 1 across the pattern on the sample 3 at a high rate in the direction indicated by the arrow. The broken line indicates the state during the blanking period of the electron beam. The deflector 4 scans the electron beam across 512 pixels in one excursion, 100 MHz per pixel.
The sample 3 is placed on a stage 5. The velocity of movement of the stage 5 and its position are controlled at an accuracy of tens of nanometers, using a linear encoder or a laser interferometer.
Because the sample 3 is irradiated with the electron beam 1, secondary electrons having energies from 0 to less than 10 eV are produced. Since a bias voltage is applied between the sample 3 and the electromagnetic lens 2, the secondary electrons are accelerated to 19 keV and enter a Wien filter (not shown) mounted over the electromagnetic lens 2. This filter does not deflect the incident electron beam 1 but deflects the secondary electrons from the sample through about 90xc2x0.
The secondary electrons deflected by the Wien filter are detected by a secondary electron detector 6. A Schottky barrier diode detector that is a high-speed secondary electron detector responding at 100 MHz per pixel is used as the detector 6. An amplifier 7 amplifies the signal at a response frequency of 100 MHz.
The output signal from the amplifier 7 is supplied to an A/D converter 8 and converted into a digital signal. The output signal from the A/D converter 8 is fed to a signal distributor 9. The stage 5 is continuously moved in the Y-direction. The electron beam 1 is scanned only in the X-direction perpendicular to the Y-direction. The resulting signal representing 512xc3x97512 pixels provides a SWATH image in synchronism with a sync signal from the linear encoder or laser interferometer in the control system for the beam deflector 4 and for the stage 5. The SWATH image is distributed to plural high-speed image processing circuit boards 10.
Each of the high-speed image processing circuit boards 10 has an image distributor 11, an image memory 12, a defect feature extractor 13, and a defect detector 14. The image distributor 11 stores the SWATH images in the image memory 12 according to given image position information. The SWATH images are collected at the timing of image collection in synchronism with the sync signal from the linear encoder or laser interferometer in the control system for the beam deflector 4 and for the stage 5.
The defect feature extractor 13 is loaded with an algorithm specialized by the signal stored in the image memory 12. Features of the image are extracted according to the signal stored in the image memory 12 in accordance with the algorithm. The defect detector 14 compares defect features of two images extracted in succession (cell comparison) or compares different chip patterns (die comparison). Alternatively, the detector compares defect features with CAD design data about a chip pattern already collected from good products or chip patterns (data comparison). Defect sizes and their coordinates are detected by a specially prepared algorithm.
A defect inspection result processor 15 integrates defect sizes, coordinates, and wafer information (e.g., wafer names, lot numbers, and wafer recipe) processed in parallel at high speed by the high-speed image processing circuit boards 10 and stores the results.
The defect inspection system shown in FIG. 1 has been developed only to focus the electron beam of the prior art scanning electron microscope as finely as possible. Therefore, where a contact hole not only has a small aperture diameter but also has a much larger depth than the aperture diameter, difficulties take place. For example, it is impossible to detect a contact hole with defective aperture if the aperture diameter is less than 0.05 xcexcm and the aspect ratio is 10 to 15.
If a contact hole in SiO2 film formed on a silicon (Si) substrate or a contact hole in a resist film is inspected with an electron beam, the accelerating voltage of the electron beam incident on the sample is set less than 1 kV where the secondary electron emission rate xcex4xe2x89xa71 to avoid charging of the oxide film or resist film. Also, it is known that the accelerating voltage is set to 0.5 to 0.8 kV in use to minimize the damage to the sample due to the electron beam bombardment.
In some inspection tools, the sample is scanned with an electron beam at an accelerating voltage of 0.8 kV at a high frequency of 100 MHz to suppress charging of the sample. In other inspection tools, the DC voltage between an objective lens and a wafer sample is adjusted to suppress charging of the sample.
The electron beams used in the method of preventing charging are finely focused to diameters of several nanometers to tens of nanometers at an accelerating voltage of 0.8 kV in the same way as in ordinary scanning electron microscopy. In this case, the aperture angles (the value of the angular aperture) of the electron beams are on the order of 10xe2x88x923 rad to 10xe2x88x922 rad. In this way, electron beams used in normal scanning microscopes take a conic form.
Where such an electron beam is scanned over a contact hole 22 having a diameter of 0.1 xcexcm and a depth of 1 to 1.5 xcexcm (aspect ratio of 10 to 15) and formed in SiO2 or resist 21 formed on an Si substrate 20, as shown in FIG. 2, charging occurs on the inner wall of the contact hole 22.
This is next described by referring to FIG. 3. The electron beam 1 impinging on the sample obliquely hits the side surface of the inner wall of the contact hole 22, producing a large amount of secondary electrons. This gives rise to positive charge. In particular, since the aperture angle (the value of the angular aperture) of the electron beam is as large as on the order of 10xe2x88x923 to 10xe2x88x922 rad, the electron beam assumes a conic form. The obliquely impinging electrons hit the side surface of the inner wall. Furthermore, the scanned beam is not strictly vertical to the sample surface, or the beam is not strictly parallel to the side surface of the inner wall of the contact hole. As a result, the trajectory of the secondary electrons se emitted from the bottom of the contact hole 22 is deflected by the electric charge on the side surface of the contact hole 22 and absorbed into the side surface. Hence, these electrons cannot escape into the opening over the contact hole 22.
A scanning electron microscope (SEM) image was obtained by the aforementioned scanning of the electron beam. The contrast of the image in the direction parallel to the contact hole is shown in FIG. 4. Variations in the signal in the direction of the cross section are shown in FIG. 5. In FIG. 4, indicated by A is the opening in the contact hole. A region B is displayed brightly because a number of secondary electrons are emitted by the edge effects. A region C is displayed darkly because the secondary electrons from the contact hole 22 are trapped in the sidewall of the contact hole. In FIG. 5, the horizontal axis indicates the scan position of the electron beam. The vertical axis indicates the intensity of the secondary electron detected.
This phenomenon gives rise to the following result. Where the aperture diameter of the contact hole 22 is greater than 0.1 xcexcm, or where the aspect ratio is 3 to 5, a relatively narrow area of the side surface of the contact hole 22 is illuminated with obliquely incident electrons. The amount of positive charge on the side surface is smaller.
Therefore, the secondary electrons from the bottom of the contact hole 22 can escape into the upper opening. Consequently, the opening of the contact hole can be inspected with this conic electron beam. However, the detection limit is an aspect ratio of about 3 to 5 where the diameter is 0.1 xcexcm. Furthermore, the charging of the contact hole is unstable and so the detection percentage of the opening inspection is as low as less than 80%.
The contrast-generating mechanism, where residues of SiO2 or resist are present at the bottom of a contact hole, is described by referring to FIGS. 6-8. FIG. 6 shows the manner in which the contact hole is charged. Like components are indicated by like reference numerals in both FIGS. 3 and 6. The residues of SiO2 or resist are indicated by 23.
Where the contact hole 22 having the residues 23 is scanned with the conic electron beam 1, the obliquely incident electrons strike the side surface of the contact hole and the residues, inducing secondary electrons. The side surface of the contact hole 22 and the residues 23 become charged positively. Because of the oblique incidence of the beam, the side surface of the contact hole produces a larger number of secondary electrons than the residues 23. In consequence, the side surface is charged to a higher positive voltage than the residues 23.
As secondary electrons emitted from the residues 23 go upward, they are accelerated upward obliquely by the positive charge on the side surface. The positive charging voltage on the sample surface close to the contact hole is lower than the voltage on the side surface because of nearly vertical incidence of the beam. The secondary electrons accelerated upward obliquely as described above are decelerated near the surface of the contact hole 22 but can escape to the secondary electron detector.
This phenomenon gives contrast to the obtained SEM image in the direction of the plane of the contact hole. This contrast is shown in FIG. 7, where A indicates the opening of the contact hole. A region B appears bright because a large number of secondary electrons are emitted by the edge effects. D indicates the bottom of the contact hole 22. Since many secondary electrons are produced from the bottom D, it appears bright. Variations in the signal intensity in the direction of the cross section of the contact hole 22 are shown in FIG. 8.
In the above-described case, the aspect ratio assumes relatively small values of about 3 to 5. Where the aspect ratio of the contact hole increases to 10 to 15, the side surface and the remaining film are charged positively in the same way as the foregoing. However, the side surface is charged to a higher positive voltage than where the aspect ratio takes small values of 3 to 5 because a larger amount of the electron beam obliquely impinges on the side surface of the contact hole, producing a larger number of secondary electrons.
Accordingly, secondary electrons produced by the remaining film are accelerated obliquely upwardly by the positive electric charge on the side surface as they go upward. The trajectory is deflected by the higher positive charge voltage on the side surface. Consequently, the secondary electrons collide against the side surface of the contact hole.
The result is that the secondary electrons emanating from the remaining film is unable to escape to the surface of the contact hole. The contrast in the direction of the plane of the contact hole is different from the contrast shown in FIG. 7. Rather, the inside of the bright annular portion that is made bright by the edge effects is darker as shown in FIG. 4, because the secondary electrons cannot escape from the opening of the contact hole. If the remaining film is exorbitantly thick and thus the effective aspect ratio is as small as about 3 to 5, the condition will be the same as the condition shown in FIGS. 7 and 8. However, if the remaining film is less than tens of nanometers (i.e., the aspect ratio is 10 to 15), the inside becomes darker. This demonstrates that increasing the aspect ratio of the contact hole makes it more difficult to obtain secondary electron contrast corresponding to the thickness of the remaining film.
In view of the foregoing, the present invention has been made.
It is an object of the present invention to provide a defect-review SEM for inspecting defective openings in small holes (e.g., having diameters of less than 0.05 xcexcm and having high aspect ratios of 10 to 15, for example) with high throughput.
It is another object of the present invention to provide a method of inspecting contact holes as described above by a defect-review SEM with high throughput.
A defect-review SEM, in accordance with the present invention, comprises an electron gun for producing an electron beam, an objective lens for focusing the electron beam onto a sample such that the beam hits the sample at a convergence angle to thereby produce secondary electrons or backscattered electrons, upper- and lower-stages of deflectors for scanning the electron beam across the sample in two dimensions, an angular aperture control lens for controlling the convergence angle, i.e., the angular aperture of the electron beam, and a detector for detecting the secondary electrons or backscattered electrons. The electron beam is scanned while vertical incidence on the sample is maintained by the upper- and lower-stages of deflectors.
The convergence angle of the electron beam impinging on the sample is minimized by the angular aperture control lens. The beam scans the sample while kept vertical to the sample by the upper- and lower-stages of deflectors.
The secondary electrons or backscattered electrons emanating from the sample are detected by the detector.
In one embodiment of the invention, the convergence angle of the electron beam hitting the sample is set to 10xe2x88x925 to 10xe2x88x926 rad. The angular aperture control lens can control the convergence angle of the beam hitting the sample from a large angle to a small angle. In the observation mode, the convergence angle is switched to a large value. In the inspection mode, the convergence angle is switched to a small value.
The defect-review SEM is further equipped with a doughnut-like detector for detecting the secondary electrons or backscattered electrons induced by the bombardment of the electron beam. This detector is located behind the objective lens on the optical axis of the electron beam.
In another feature of the invention, an electric field control lens is mounted between the sample and the objective lens. The voltage applied between the sample and the field control lens can be adjusted within a range from 0 to xc2x11 kV.
In a method of inspecting numerous contact holes in a sample in accordance with the present invention, the sample is illuminated with an electron beam at a convergence angle of 10xe2x88x925 to 10xe2x88x926 rad. The beam is scanned in two dimensions while kept vertical to the sample. Resulting secondary electrons or backscattered electrons are detected by a doughnut-like detector mounted on the optical axis of the electron beam behind the objective lens.
A reference sample is used in the defect-review SEM, in accordance with the present invention, to adjust the convergence angle of the electron beam and the vertical incidence. The reference sample has a groove or hole having an inner vertical wall. The reference sample has scales located at different heights.
The groove having the inner vertical wall is formed by mounting two members having vertical side surfaces via a gap between them such that their vertical side surfaces face each other. Alternatively, grooves having inner vertical walls and extending in different directions may be formed.
In a further embodiment of the invention, the reference sample comprises four rectangular parallelepipeds having vertical side surfaces and scales located at different heights. The four rectangular parallelepipeds are arranged in two rows and two columns via given spaces between them in a two-dimensional manner. Using this reference sample, the convergence angle and vertical incidence of the electron beam are adjusted.
In addition, the four rectangular parallelepipeds having the vertical side surfaces are cut from a (100) single crystal of Si or GaAs. The scales consist of pieces of mesh of Ni or gratings. Each cut parallelepiped has a thickness of about 0.6 to 0.7 mm and is about 10 mm square on the plane. The space between the four parallelepipeds is 0.1 to 0.2 mm. The pieces of mesh are of to 100 xcexcm mesh. One of the heights at which the scales are located is flush with the height of the top surfaces of the parallelepipeds, while the other height is flush with the height of the bottom surfaces of the parallelepipeds.
Other objects and features of the invention will appear in the course of the description thereof, which follows.